JPH02225301A - Oxide superconductor - Google Patents

Abstract

PURPOSE:To obtain an oxide superconductor exhibiting superconducting phenomenon at a temperature higher than liquid nitrogen temperature and having excellent stability to water by having specific composition of rare earth-alkaline earth metal-Cu-O system and having A3B2O6 crystal structure containing two-dimensional plane. CONSTITUTION:A complex oxide having composition expressed by the formula RE2-xMII1+x-yMIyCu2O6+delta (RE is La, Pr or Nd; MII is Sr or Ca; MI is K or Na; 0<=x<=1, preferably x is 0-0.5; 0<=y<=1, preferably y is 0.1-0.5; delta is oxygen defect) and having A3B2O6 crystal structure containing two-dimensional plane of Cu-O is used as an oxide superconductor.

Description

Detailed Description of the Invention [Object of the Invention] (Industrial Application Field) The present invention relates to a novel superconductor using a rare earth-alkaline earth metal-Cu-0 based oxide.

(Prior art) Many superconducting materials whose electrical resistance becomes zero below a certain temperature have been discovered, and various attempts have been made to use them to create superconducting wires and superconducting electronic devices. It's coming.

In addition, various so-called high-temperature superconductors having transition temperatures higher than the boiling point temperature of liquid nitrogen have recently been discovered, and attempts to realize superconducting wires and superconducting devices that do not require expensive liquid helium have attracted interest. The above-mentioned high temperature superconductor includes a defective perovskite oxide superconductor having oxygen defects represented by Y-Ba-Cu-O system,
Furthermore, oxide superconductors such as B1-8r-Ca-Cu-0 series and Tl-Ba-Ca-Cu-0 series, which have higher critical temperatures, have been discovered.

On the other hand, general formula: RE2Ml, Cu2o, δ------
-・-(1) (wherein, RE is at least one selected from La, Pr and Nd, and Ml is Sr and Ca.
At least one selected from: same as below. The oxide represented by It is a substance that is expected to have a high possibility of becoming a human body. However, although it exhibits metallic conductivity, there have been no reports to date of superconducting phenomena up to 4.2K.

This is 1. Considering La Sr Cu O, if 2 1 2 60 δ and the composition of La:Sr:Cu = 2:1:2 and oxygen is 6, the conductivity will decrease because the formal charge of Cu is 2. Not shown. Therefore, conductivity is obtained by introducing oxygen for +5 minutes, but this +5 minutes oxygen makes Cu
Since the two-dimensional flatness of the −0 plane is destroyed, it is thought that the superconducting phenomenon does not occur.

In addition, it is possible to replace a part of La with Sr in the solid solution region of La and Sr, which allows Cu
Attempts have also been made to increase the formal charge of the material to obtain conductivity, but in this case, the crystal structure itself transforms into an insulator structure with a different arrangement of oxygen, so superconductivity has not been achieved. do not have.

(Problems to be Solved by the Invention) As described above, since the rare earth-alkaline earth metal-Cu-0 based oxide represented by the above formula (1) has a two-dimensional Cu-0 state, It is expected to be a new superconductor, but so far no superconducting state has been demonstrated.

Further, the oxide represented by the above formula (1) is Y-Ba-C
The rare earth-alkaline earth metal-Cu-0 based oxide uses Sr and Ca, which are relatively stable compared to the alkaline earth elements Ba used in the u-0 based oxide superconductor. If a superconducting state is achieved by this method, it can be expected to overcome the drawback of Y-Ba-Cu-0-based oxide superconductors that they are unstable with respect to moisture.

The present invention was made in order to deal with such problems, and it is said that 2
126+δ C of the above rare earth-alkaline earth metal-Cu-0 based oxide
Provides conductivity without destroying the two-dimensionality of the u-0 plane,
The objective is to provide an oxide superconductor that achieves a superconducting state.

[Structure of the invention] (Means for solving the problems) The oxide superconductor of the present invention has the following general formula: RE2□MN, -, Mj, Cu2OB+
6-(II) (In the formula, I'll': represents at least one selected from La, Pr and Nd, and Ml represents Sr.
and at least one selected from Ca, MI is K
and represents at least IFi selected from Na and X
satisfies 0≦X≦1, y is a number that satisfies Q<y≦1, and δ represents an oxygen defect and is usually a number from 0 to 0.2. same as below. ) and has a composition represented by C
It is characterized by having a ^lB2O6 crystal structure with a two-dimensional u-0 plane.

That is, the present invention replaces a part of +2 valent monoatomic atoms C8r and Ca) in the above formula (I) with +1 valent) III atoms (K and Na) whose ionic radius is close to these, and By setting the charge to 2 or more, conductivity is imparted without destroying the Cu-0'p plane, making it possible to realize a superconducting state.

As mentioned above, if the values of X and y in the above equation (n) satisfy 0≦x≦1 and 0<y51, respectively, it is possible to achieve a superconducting state.
From the viewpoint of the transition temperature to a superconducting state and reproducibility, it is preferable that X be a number greater than O and 0.5 or less, and y be a number from 0.1 to 0.5.

Examples of methods for producing the oxide superconductor of the present invention include the following two methods.

The first method is a method using a melt excitation method.

That is, first, among the metal elements constituting the oxide superconductor of the present invention, a single substance or a compound of rare earth elements (RIE elements), alkaline earth elements (M1 elements), and Cu, excluding alkali elements (M1 elements), is sufficiently dissolved. Mix with Compounds of these constituent elements include La2O3, SrCO
3. In addition to oxides such as CuO and carbonates, compounds other than carbonates, such as nitrates and hydroxides, which are converted to oxides after firing, organic acid salts, organic metal compounds, and the like may be used.

These rare earth elements (R2 elements), alkaline earth elements (M
1 element) and Cu are basically mixed so as to satisfy the atomic ratio of the following formula (1), but they may be slightly deviated depending on the manufacturing conditions.

""2-X "1.+X"2OB+6
......-(III) Next, this mixed powder is calcined in the atmosphere at a temperature of 850°C to 9511r'C, and this calcined product is pulverized using a ball mill, sand grinder, or other known method. do.

This calcined powder is then treated with a compound of MI origin, such as NaC.
l, chloride such as MCI is used as a flux and melted. Melting temperature is 12O0'c ~ 1500
It is about ℃.

Thereafter, the melt is slowly cooled to precipitate oxide superconductor crystals. During the precipitation of this oxide superconductor crystal, some of the po atoms in the calcined product are substituted with )11 atoms in the flux, and 14I atoms are introduced into the crystal structure, resulting in the above (, II
) An oxide superconductor represented by the formula is obtained.

The mixing ratio of the above calcined material and flux when producing this melt is H! Set amount of atoms introduced, cooling rate after melting,
The molar ratio of the above-mentioned calcined product to the flux is approximately 1:1 - 1:5, although it should be set appropriately depending on the compound used as the flux.

Note that it is also possible to form a single crystal by controlling the annealing temperature during precipitation of the oxide superconductor crystal. In the case of single crystallization in this way, the temperature is 900°C from the melting temperature.
It is necessary to cool the crystals sufficiently slowly within the crystal precipitation temperature range, and after that, it is preferable to create an atmosphere that allows oxygen to be incorporated into the crystals.

Moreover, the second method is a method using a sintering method. That is, first, in addition to the simple substances and compounds of each metal constituent element used in the first method above, a simple substance and compounds such as oxides of the )I element are used to satisfy the atomic ratio of the above formula (n). Mix. Note that this mixing ratio may be slightly different depending on manufacturing conditions and the like.

Next, this mixed powder is once calcined at about 50° C. to 950° C. to substantially form the crystals of the oxide superconductor represented by the above formula (n) through a solid phase reaction. Next, after pulverizing this calcined product, a molded body having a desired shape is produced by a known molding method such as a press molding method. Thereafter, this molded body is fired and sintered in an oxygen-containing atmosphere at a temperature of 1000° C. to 1200° C., thereby obtaining an oxide superconductor represented by the above formula (II).

Further, after this firing, heat treatment such as oxygen annealing or reduction annealing at a relatively low temperature is performed as necessary.

Note that the method for producing an oxide superconductor of the present invention described above can be carried out using various forms of the starting materials and the sintered body.

For example, the starting material or oxide superconductor powder may be made into a paste and screen printed onto a substrate, and the material formed in the desired pattern may be fired, or the starting material or oxide superconductor powder may be It is also possible to sinter the wire after sealing it in other metal tubes and drawing the wire.

(Function) In the oxide superconductor of the present invention, the A3B2 shown in the above formula (1), which has been thought not to exhibit superconducting phenomenon,
M1, which is a divalent metal in an oxide that makes the O6 crystal structure H
A part of one atom (Sr and Ca) is a monovalent M! whose ionic radius approximates these! Atoms (K and Na) are substituted, and the formal valence of Cu is 2 or more. As a result, holes are introduced, and in a state where +6 oxygen is sufficiently small, conductivity can be imparted while maintaining good two-dimensionality of the Cu-0 plane. As described above, the oxide superconductor of the present invention has a Cu-0 plane with good two-dimensional planarity and is an oxide with a novel structure having holes introduced by Ml atoms. It exhibits superconducting phenomenon above liquid nitrogen temperature.

Moreover, the oxide superconductor of the present invention is similar to the conventional Y-Ba-C
Unlike u-0 type oxide superconductors, Sr and Ca, which are relatively stable as upper alkaline elements, are used, and because they have high sinterability, they can be obtained as dense products, so they are resistant to moisture. Maintain stable condition.

(Example) Next, embodiments of the invention will be described.

Example 1 First, as starting materials for RE elements, static elements, and Cu, La
2O3, SrCO3, and CuO powders were weighed so that the molar ratio of cations was La:Sr:Cu=1.9:1.1:2, and after thoroughly mixing them in a ball mill, This mixed powder was calcined in air at 900° C. for 24 hours to undergo a solid phase reaction.

Next, this calcined product is thoroughly ground again in a ball mill, and the weight ratio of this calcined powder and NaCl as a flux is 1.
A predetermined amount was weighed out so that 1, and the mixture was sufficiently mixed in a ball mill, and then heated and melted in air at 1300° C. for 12 hours to produce a uniform melt.

Thereafter, this melt was slowly cooled to 900°C over 42 hours, and then cooled in a furnace from 900°C to room temperature.

The obtained cooled and solidified product contains approximately 4 am x 4 sm x 4
Eleven oxide-based single crystals were precipitated.

When we analyzed the composition of the single crystal obtained in this way, we found that
It was found that Na was dissolved in solid solution, and the composition was confirmed to approximately satisfy the following formula.

La b9sr o, aNa o, acu2O e Also, it was confirmed by X-ray diffraction that the crystal structure had an A3 B2O6 structure.

When the magnetic susceptibility of this single crystal was measured, it showed Meissner, confirming that this single crystal was an oxide superconductor.

In addition, this oxide superconductor tli crystal was heated at a relative humidity of 80%.
After being left under these conditions for two weeks, the magnetic susceptibility was similarly measured, and the volume fraction showed nearly 100% Meissner, confirming that it had good durability against moisture. .

Example 2 The calcined powder produced in Example 1 above and N as a flux
Except for adjusting the mixing ratio with aCl to 1:3 by weight,
An lli crystal of the oxide was produced under the same conditions.

When the 111 crystal thus obtained was also subjected to compositional analysis and crystal structure identification, it was confirmed that it had a ^3B2O6 crystal structure and that its composition approximately satisfied the following formula.

La 1. gSr Q, 6''o, 5cu2Oe This single crystal also showed Meissner according to the constant magnetic susceptibility, and was confirmed to be an oxide superconductor.

Comparative Example After pulverizing the calcined product expressed by the following formula produced in Example 1 above, it was press-molded into a shape of 1Osa It was sintered by firing at 1050°C for 24 hours.

La14 Sr1. , I Cu2Oa, δ When the electrical resistance of the sintered body thus obtained was determined to be 1lP1, it showed metallic conduction but no superconductivity phenomenon up to 4.2K. Moreover, the magnetic susceptibility showed a positive value.

[Effects of the Invention] As is clear from the above examples, the oxide according to the present invention is a novel oxide superconductor that exhibits a superconducting phenomenon above the liquid nitrogen temperature, and is It has better stability against moisture than 0-based oxide superconductors, and can be said to be a material suitable for applications that utilize the 6-volatile superconducting phenomenon.

Applicant: Toshiba Corporation

Claims (1)

[Claims] (1) General formula: RE_2_−_xM^II_1_+_x_
−_yM^ I _yCu_2O_6_+_δ (in the formula, R
E is at least one selected from La, Pr and Nd, and M^II is at least one selected from Sr and Ca.
species, M^ I represents at least one species selected from K and Na, x satisfies 0≦x≦1, and y satisfies 0<y≦1.
is a number that satisfies the following, and δ represents an oxygen defect. ) and an A_3B_2O_6 crystal structure with a Cu-O two-dimensional plane.